Sir Ian McKellen has a reputation for taking his clothes off on stage — those who saw his Lear in 2007 may still have sleepless nights over what his understudy memorably called that “magnificent manhood dangling in the dusty air” — but mercifully he stopped short of a full reveal when he stripped at a charity event for the Park Theatre, in north London, the other day. McKellen came head to toe in Arsenal FC kit and gradually removed his hat, shirt, shorts, etc, to auction the items. To Benedict Cumberbatch, who bought the scarf for £50, McKellen reflected that when he was young he had looked like the Sherlock star, above right. “Unfortunately, you now know what you are going to look…